This invention relates to an electrical sensor which is suitable for sensing properties of liquids, in particular chemical properties, and which can be used in an environment in which it may be exposed to non-aqueous liquids, and to the use of such a sensor in an environment such as an oil well.
Various different electrical sensors are already known, for example glass electrodes for measuring pH, and resistance thermometers for measuring temperature. Glass electrodes are one type of electrochemical sensor; their operation depends upon there being contact between the glass electrode and an aqueous phase, so that problems can arise if such electrodes are used in environment such as an oil well where they may be exposed to non-aqueous liquids. Sensors used in such an environment may also become fouled. Cleaning techniques which are suitable for use at ambient pressures may not be effective at the high pressures experienced at depth in such a well.
According to the present invention there is provided an electrical sensor module, the module comprising at least one electrochemical sensor for a chemical species, a micro-porous barrier to separate the or each electrochemical sensor from the environment of the module, the micro-porous barrier being of a material which is readily wetted by water, and electrolytic means for cleaning the micro-porous barrier.
The micro-porous barrier might be of electrically conducting material, for example micro-porous steel in the form of a membrane. In this case the cleaning means may comprise an electrically conducting mesh adjacent to the membrane but spaced apart from it, and means to provide a brief electrical voltage pulse between the mesh and the membrane, at intervals, so generating micro-bubbles by electrolysis which displace any fouling material from the surface of the membrane.
Alternatively the micro-porous barrier might be of a non-electrically conducting material, for example glass frit. In this case the cleaning means may comprise electrodes at either side of the barrier, and means to provide a brief electrical voltage pulse between the electrodes, at intervals, so generating micro-bubbles by electrolysis which flow through the glass frit and emerge at its exposed surface, dislodging any fouling material. The width of the micro-porous barrier separating the electrodes is desirably no more than 5 mm to ensure that the entire width of the surface is cleaned. The barrier might be in the form of an annulus, with concentric electrodes. A plurality of such barriers might be provided, separated from each other by electrodes, and the brief electrical pulses need only be provided between the outermost electrodes, as electrolysis will occur at the intervening electrodes which will act in a bipolar fashion.
The sensor module preferably comprises a plurality of non-liquid electrochemical sensors, for example a solid state pH electrode, a solid state chloride-ion sensing electrode, and a reference electrode. The module may also comprise a temperature sensor such as a platinum resistance thermometer. The reference electrode may comprise a second solid-state chloride-ion sensing electrode, coated with a gel or polymer containing a substantially constant concentration of chloride ions; this must be in electrical contact with the environmental liquids, but a mechanical barrier such as a perforated PTFE (Goretex) membrane or an impermeable barrier with a small hole may be provided to restrict diffusion of chloride ions away from the gel or polymer.
It will be appreciated that the electrolytically cleanable glass frit barrier described above could be used in other situations.